Remove the implementation of the `FileCollectionInternal` interface in
`KotlinCompilationOutput`. This interface may change unexpectedly (and
does between Gradle 4.10 and 5.0) so we need to get rid of its
implementation.
On the other hand, we can't just implement `FileCollection` instead
because Gradle internally assumes that all `FileCollection` instances
are also `FileCollectionInternal`. So the way to fix this is to remove
`SourceSetOutput` from superinterfaces of `KotlinCompilationOutput`
altogether.
Instead, to work with `SourceSetOutput` and `KotlinCompilationOutput`
in a uniform way, provide a wrapper for `SourceSetOutput`:
`KotlinWithJavaCompilationOutput` that is basically a proxy
class.
Sergey Igushkin
a0f4d5a637